A polyester-based adhesive (this may be referred to as a pressure-sensitive adhesive—the same shall apply hereinafter) is typically formed by applying a liquid composition (polyester-based adhesive composition), which has been prepared by dissolving or dispersing an adhesive-forming component containing a polyester and its crosslinking agent in an organic solvent, onto a substrate, followed by drying and curing (crosslinking) the composition. Recently, from the viewpoint of environmental considerations, production costs and others, the request for refraining from using organic solvents (this may be referred to as organic mediums) in the production process for adhesive compositions and/or pressure-sensitive adhesive sheets is increasing. A polyester-based adhesive composition does not indispensably require using an organic solvent in synthesis (through polycondensation) of the polyester, and is therefore expected to satisfy the above-mentioned requisition.
In general, however, a polyester suitable for the use for adhesives has a relatively high molecular weight and therefore the viscosity thereof in a temperature range around room temperature is too high for coating. Accordingly, in an ordinary conventional polyester-based adhesive composition, the polyester is diluted with an organic solvent to improve the coatability with the composition. A polyester having a lower molecular weight tends to have a lower viscosity, but in conventional technology, it has been difficult to realize an adhesive capable of exhibiting sufficient adhesion performance (especially adhesive force) by the use of a polyester having a low molecular weight to such a level that could attain good coatability even without diluted with an organic solvent.
Patent References 1 and 2 relate to a polyester-based adhesive composition and describe a technique that satisfies both coatability and adhesiveness of a solvent-free composition. However, the cured product (adhesive) formed of such composition is poor in tackiness (stickiness) in a room temperature range and therefore could not be pressure-bonded to an adherend (that is, could not produce adhesive force) at around room temperature, and accordingly, the composition is unsuitable to an adhesive.